Vehicles have a cooling device that suppresses overheating and overcooling of the engine so as to maintain the temperature of the engine at an appropriate level. A water cooling type cooling device, which cools the engine by circulating coolant water in the interior of the engine, has a water jacket, that is, a coolant water passage extending in a cylinder block and a cylinder head of the engine. The coolant water is circulated through the water jacket by means of a water pump so as to absorb the heat of the engine. The coolant water, which has been heated to a high temperature by the heat of the engine, is then sent to a radiator, which is a heat exchanger. The coolant water is thus cooled by the air blowing through the radiator and returned to the water jacket.
When the engine must be warmed up, such as immediately after the engine is started, the temperature of the coolant water is quickly elevated to an appropriate level by stopping circulation of the coolant water via the radiator. However, since the water pump typically interlocked to the crankshaft, it is impossible to stop the operation of the water pump. That is, it is impossible to stop the circulation of the coolant water as long as the engine is in operation. To solve this problem, the cooling device for a vehicle has a bypass passage that allows the coolant water to bypass the radiator when circulating. Accordingly, when the engine warms up, the cooling device quickly raises the temperature of the coolant water to the appropriate level by circulating the coolant water via the bypass passage.
Switching of the circulation paths of the coolant water between the path for engine warm-up and the path for after completion of the engine warm-up, which has been described, is performed by a thermostat, which is a temperature sensitive valve that operates in response to the temperature of the coolant water flowing into the valve. Conventionally, for this purpose, various types of thermostats have been proposed and used as described in, for example, Patent Documents 1 to 4. Typical conventional thermostats have the basic configuration described below. Specifically, a thermostat includes a valve body that is moved by thermal expansion and thermal contraction of a substance sealed in a thermo-element, which is a temperature sensitive portion. A wax pellet type thermostat, for example, employs a bullet-like container in which wax is sealed as its temperature sensitive portion. The wax, which is a solid under low temperatures, melts and expands under high temperatures, thus moving the valve body of the thermostat. This selectively opens and closes the valve in such a manner that the coolant water is circulated via the bypass passage when the temperature of the coolant water is low but through the radiator when the temperature of the coolant water is sufficiently high.
The coolant water that has been heated by the engine is used by a heater that raises the temperature in the passenger compartment. In other words, after having been heated by the engine, the coolant water is sent also to a heater core, which is a heat exchanger, and used by the heater core to warm the air that is blown into the passenger compartment.
A cooling device for a vehicle employing the water cooling, which has been described above, has a plurality of passages in which the coolant water circulates. Since the coolant water passages have complicated structures, a large number of components and steps are necessary for formation of the passages. Accordingly, it has been demanded that the configuration of passages for the coolant water be simplified to save the manufacturing costs.    Patent Document 1: Japanese Laid-Open Patent Publication No. 02-146219    Patent Document 2: Japanese Laid-Open Patent Publication No. 08-319828    Patent Document 3: Japanese Laid-Open Patent Publication No. 10-019160    Patent Document 4: Japanese Laid-Open Patent Publication No. 2006-37889